The present invention relates to a tracking cathode ray tube circuit, comprising tracking means connected to a shadowmaskless tracking cathode ray tube for deriving a tracking signal therefrom.
Such a tracking cathode ray tube circuit, which is also referred to as index tube, is known from GB-1 403 061. Disclosed therein is a so-called shadowmaskless tracking cathode ray tube (CRT), wherein a feedback control loop controls the exact landing spot of electron beams on a front plate thereof. The position of the spot is measured, using a guiding track structure or sensor positioned on the inner side of the front plate. When a spot is scanned along the tracks, a tracking signal which depends on the spot position arises, which tracking signal can be used to generate a spot position output signal for a deflection unit on the tracking CRT which keeps the spot on track. The guided track structure can be designed in such a way that a modulated tracking signal, which is at on a very high voltage relative to earth, is generated. A guiding track structure having a constant wobble pitch is known to result in an error signal which is constituted by an amplitude-modulated signal at a carrier frequency. The error signal can then be discriminated from the video modulation without confusion.
It is, however, a disadvantage of the known shadowmaskless tracking cathode ray circuit that, apart from the shape of the tracking structure, the modulation frequency and the modulation depth have to be chosen correctly in relation to the video modulation in order to prevent artefacts, which are undesirably visible on the screen of the CRT.
It therefore is an object of the present invention to provide a cathode ray tube circuit which has a less critical structural design and internal shape and does not give rise to artifacts or necessitate fine tuning of internally mounted and prescribed modulation quantities and features.
To this end, the tracking cathode ray tube circuit according to the invention is characterized in that the tracking means comprises a modulator-demodulator circuit. It is an advantage of the tracking CRT circuit according to the invention that, internally, the tracking hardware of the shadowmaskless tracking CRT tube requires no adjustment, because an unmodulated/base band tracking signal will result from the shadowmaskless tracking CRT tube in the circuit according to the invention. The tracking means can now be simply provided outside the tracking CRT tube, while comprising modulator demodulator means for creating a modulated tracking signal therewith. Such a modulated tracking signal can be advantageously and easily down converted from anode potential (typically 25 kV) to earth potential.
An embodiment of the tracking CRT circuit according to the invention, which is easy to implement, is characterized in that the modulator, which is connected to the shadowmaskless tracking cathode ray tube, and the demodulator are mutually separated by a first AC coupling. Such an AC coupling may be advantageously of a conventional type, for example, an electric, magnetic, optical or acoustical coupling.
A further embodiment of the tracking CRT circuit according to the invention is characterized in that the tracking means comprises an oscillator which is coupled to both the modulator and the demodulator. It is an advantage of the tracking CRT circuit according to the invention that only one oscillator is necessary for driving the modulator as well as the demodulator.
A further embodiment of the CRT circuit according to the invention is characterized in that the oscillator is connected to the modulator and is coupled to the demodulator through a second AC coupling. It is an advantage of this embodiment of the tracking CRT circuit according to the invention that possible frequency and phase problems are obviated, because particularly the phase differences across both AC couplings for the tracking signal and the oscillator signal, respectively, will be equal. This provides the possibility of a simple and correct synchronous detection in the demodulator.
A further embodiment of the index CRT control circuit is characterized in that the oscillator is coupled to a DC/AC converter, which in turn is coupled to a DC supply source and whose converter output is coupled to the second AC coupling. In this embodiment, the AC coupling concerned simultaneously provides a supply voltage and the modulated or modulating signal to the demodulator or modulator respectively without additional circuitry being required.
Advantageously,several variants and embodiments of the tracking CRT circuit are feasible. The AC coupling may again be of a conventional type, such as, for example, a capacitive, inductive, optical or acoustical coupling. The modulation, which will generally be analog, may comprise a variety of modulation types, such as: AM, FM, PM, DSB, SSB, VSB, FSK, etcetera.
In a further preferred embodiment, in which the AC coupling is an inductive coupling, this coupling is advantageously arranged as an additional tap on a high-voltage transformer of a display device, such as for example, a television apparatus. This economizes on complete inductive coupling.
Another embodiment of the shadowmaskless tracking CRT circuit is characterized in that the demodulator comprises an AM detection circuit and a phase comparator for providing information about the displacement of the electron beam from the centre of the track and about the direction of the displacement, respectively. This embodiment provides detailed information about displacement, velocity and heading of the separate respective electron beams in question.
The present invention also relates to a display device, such as, for example, a television apparatus, comprising a tracking cathode ray tube circuit as defined in the claims in which the cathode ray tube circuit comprises coupling means connected to a shadowmaskless tracking cathode ray tube for deriving a tracking signal therefrom, which device is characterized in that the coupling means comprises a modulator demodulator circuit.